Equilibrium model approach to predict local chemical changes in recovery boiler deposits

A step-by-step equilibrium model approach was developed to estimate and describe the enrichment of K and Cl within recovery boiler deposits. The model predicts free melt movement towards colder temperatures within deposits. Due to a temperature gradient, the melt amount and composition differ across the deposit. The melt movement affects the local composition, which leads to changes in the local phase composition. The model predicts how the deposit profile changes due to melt migration into pores within the deposits. Changes in the deposit composition profile also affect the melting behavior locally, resulting in lower local melting temperatures, which can be detrimental to heat exchanger materials. The modeling results were compared to earlier published laboratory and full-scale boiler measurements, and there is a good agreement between the results. The model predicts a local decrease in the first melting temperature of recovery boiler deposits by ∼30 °C. These findings closely align with experimental results, shedding light on the intricate mechanisms of melt percolation and intra-deposit aging processes. The proposed step-by-step model offers a means to achieve more accurate estimations of locally prevalent first melting temperatures in recovery boiler deposits.